Sim900 GSM + Arduino Mega 2560

Hello…
I follow this entire video (GSM Shield on an Arduino Mega tutorial - YouTube) on my arduino, but i’m getting DB:NO RESP error on Serial Monitor.

how can i fix it?

DB:NO RESP
DB:NO RESP
DB:NO RESP
Trying to force the baud-rate to 9600

1200
2400
4800
9600
19200
38400
57600
115200
ERROR: SIM900 doesn’t answer. Check power and serial pins in GSM.cpp

status=IDLE